1. Field of the Invention
The present invention relates to a process for continuously producing polyester and to a process for producing a spun fiber.
2. Description of the Prior Art
Linear polyesters comprising polyethylene terephthalate as a main component have generally been produced by a process which comprises producing bis-.beta.-hydroxyethyl terephthalate and/or a lower polymer thereof by esterification of terephthalic acid with ethylene glycol or an ester exchange reaction of dimethyl terephthalate with ethylene glycol, and polymerizing under reduced pressure while removing ethylene glycol.
Both a batch process, which has been used hitherto, and a continuous process which can be used instead of the batch process as a result of recent developments can be used to produce the polyesters. The principal advantages of the continuous process as compared with the batch process are that it is possible to spin the polymer produced by the polymerization reaction directly. The polymer is introduced in a melt state into a spinning step and spun by means of nozzles to produce unstretched filaments. Using the continuous process, steps such as granulating, drying and remelting, which are essential to the batch process can be omitted which decreases the cost and unnecessary residence time that does not contribute to polymerization is shortened to the point at which it is practically nonexistent. In this manner it becomes possible to produce a homogeneous polymer having good uniform quality continuously.
However, in the case of carrying out direct spinning in order to take advantage of the continuous process, non-uniform quality of the polymer produced by the polymerization reaction often causes uneveness in the quality of the unstretched filaments spun by spinning nozzles. The ability to control the quality of the polymer and particularly the degree of polymerization is important in the continuous process.
One example of a method for controlling the degree of polymerization of the polymer is by continuously measuring the melt viscosity (which is directly related to the degree of polymerization) of the polymer removed from the polymerization reactor, and controlling the vacuum in the polymerization reactor so as to keep the viscosity at a definite level. However, when a factor (such as a change in the esterification degree or a change in the feed rate) disturbs the polymerization reaction, the appearance thereof as reflected in a change in the melt viscosity of the polymer is delayed, and, if the vacuum is controlled based on the time at which the melt viscosity changes, the degree of polymerization of the polymer can not be accurately controlled due to the delay in the response time.
The present inventors have paid attention to the fact that the response time of melt viscosity of the polymer at the outlet of the reactor is very slow in the prior process when a disturbance in the polymerization reaction is encountered, which is the principal drawback in the prior control method; and to utilizing the prior control method by substituting the melt viscosity with another control medium having a fast response time.
Namely, in the batch polymerization process, the degree of polymerization of the product has been easily controlled by detecting the melt viscosity by the reaction force applied on the stirring axis (the resistance of the polymer to stirring) and removing the vacuum at the time the desired viscosity is attained (the degree of polymerization gradually increases as the vacuum in the reactor in which the lower polymer is charged increases).
In the batch polymerization process, since the melt viscosity of the polymer in the reactor is uniform, it bears a primary relation to the reaction force on the stirring axis. Further, since the melt viscosity gradually increases from several poises to several thousand poises, the change in the reaction force on the stirring axis is large and it is possible to control the degree of polymerization of the polymer by monitoring the reaction force. However, in the continuous polymerization process, the viscosity of the polymer in the reactor increases from the inlet to the outlet and, consequently, the reaction force on the stirring axis is not always directly related to the melt viscosity of the polymer at the outlet of the reactor. The reaction force on the stirring axis is dominated by the viscosity gradient. Further, in normal operation, the change in the melt viscosity in the reactor is small and, consequently, the change in the reaction force on the stirring axis is small and difficult to detect. Accordingly, it has been assumed that control of the degree of polymerization of the polymer at the outlet of the reactor by monitoring the reaction force on the stirring axis is difficult in continuous polymerization.